1. Field of the Invention
The present invention relates to a biaxial oriented polyester film, and more particularly to a biaxial oriented polyester film preferable as a base film for a magnetic tape.
2. Description of the Prior Art
A biaxial oriented polyester film has been used as a preferred magnetic recording material, especially as a base film material for a magnetic tape because of its excellent physical and chemical properties. In recent years, a film having a satisfactory abrasion resistance, more specifically, a film which does not generate abrasion powders during a calendering step in the course of the production of a magnetic tape, has been demanded along with a high speed process for producing the magnetic tape. The calendering comprises the step of smoothing the surface of a magnetic layer, the magnetic layer being obtained by coating a composition for a magnetic layer onto a polyester film, followed by drying. According to the calendering step, the polyester film passes between rollers onto which an extremely high pressure is applied, so that protrusions on the surface of the polyester film on which the magnetic layer is not formed are scraped off to generate abrasion powders.
As a method for preventing the scrapes, for example, a method in which a degree of crystallization of the base film is decreased is disclosed in Japanese Laid-Open Patent Publication No. 60-195727. However, according to this method, the following problem arises. When the base film on which the magnetic layer is formed is slitted into a predetermined width to be molded into a magnetic tape, edge damages (edge up and whisker) and powders are remarkably generated from a cutting section of the base film, resulting in a frequent dropout of the obtained magnetic tape.